Silicon carbide is hard, strong, and exhibits good corrosion and abrasion resistance and high thermal conductivity. It can be used in oxidizing temperatures up to 2500.degree. F. These properties render silicon carbide a useful material in many acid, caustic, corrosive, abrasive, or high temperature environments. Such applications include pump seals and bearings, gas turbine components, mixing nozzles, and flame holders.
Silicon carbide bodies are frequently formed by a sintering process. Sintered silicon carbide has a high hardness, good corrosion resistance, and high thermal conductivity. In sintering, particles of a material bond together when heated to a high temperature, below the material's melting point. In some processes, the material is also subjected to high pressure as well. A self-sintered process is one which does not require application of pressure during the heating step for sintering to occur.
A drawback to silicon carbide is its lack of self-lubricity. A self-lubricating solid is one having low friction in the absence of an additional lubricant. For example, in applications having a high PV (pressure-sliding velocity) limit or dry running applications, parts, such as seals, having a silicon carbide face adjoining a face made of silicon carbide, other ceramics, or steel, will wear excessively due to the forces generated by the high friction. In dry running applications with mating surfaces, special wear surfaces must be provided on at least one of the bodies.
Graphite is a known lubricant and has been incorporated into carbon and silicon carbide materials to impart a self-lubricating property to the material. However, with sintered materials, it has been difficult to incorporate large amounts of a second phase such as graphite into a ceramic matrix without causing cracks to occur in the microstructure or without increasing the material's porosity. Further, adding graphite to silicon carbide is even more difficult, because sintering of silicon carbide already requires stringent conditions, such as fine, high purity powders, sintering aids, and high temperature.
It is known to form a silicon carbide/graphite material by reaction bonding or reaction sintering. However, reaction bonded silicon carbide/graphite material typically has a residual silicon phase which limits corrosion resistance due to reaction with the silicon in some chemical applications. Also, controlling the reaction bonding process to obtain fully reacted and fully dense parts is difficult.
Another known silicon carbide material incorporating graphite is disclosed in U.S. Pat. No. 4,525,461. This material comprises a sintered silicon carbide/graphite/carbon composite ceramic body having a homogeneous fine grain microstructure. At least 50% of the silicon carbide grains are less than 8 .mu.m, and the graphite grains have an average size no larger than that of the silicon carbide grains. However, if the amount of graphite is greater than approximately 8% by weight in this material, the material's density decreases. Less than 8% by weight graphite, while providing a more dense, impervious structure, limits the graphite's lubricating capability in the material.
Thus, there exists a need for a dense, impervious self-sintered silicon carbide body with a greater amount of graphite inclusions to increase the lubricity of the material while maintaining the integrity of the microstructure.